Conformal thermal interface material for electronic components

ABSTRACT

A thermally-conductive interface for conductively cooling a heat-generating electronic component having an associated thermal dissipation member such as a heat sink. The interface is formed as a self-supporting layer of a thermally-conductive material which is form-stable at normal room temperature in a first phase and substantially conformable in a second phase to the interface surfaces of the electronic component and thermal dissipation member. The material has a transition temperature from the first phase to the second phase which is within the operating temperature range of the electronic component.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/714,680, filed Nov. 16, 2000; which is an application forreissue of U.S. patent application Ser. No. 08/801,047, filed Feb. 14,1997, now U.S. Pat. No. 6,054,198, granted Apr. 25, 2000, the disclosureof each of which is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates broadly to a heat transfer materialwhich is interposable between the thermal interfaces of aheat-generating, electronic component and a thermal dissipation member,such as a heat sink or circuit board, for the conductive cooling of theelectronic component. More particularly, the invention relates to aself-supporting, form-stable film which melts or softens at atemperature or range within the operating temperature range of theelectronic component to better conform to the thermal interfaces forimproved heat transfer from the electronic component to the thermaldissipation member.

[0003] Circuit designs for modern electronic devices such astelevisions, radios, computers, medical instruments, business machines,communications equipment, and the like have become increasingly complex.For example, integrated circuits have been manufactured for these andother devices which contain the equivalent of hundreds of thousands oftransistors. Although the complexity of the designs has increased, thesize of the devices has continued to shrink with improvements in theability to manufacture smaller electronic components and to pack more ofthese components in an ever smaller area.

[0004] As electronic components have become smaller and more denselypacked on integrated boards and chips, designers and manufacturers noware faced with the challenge of how to dissipate the heat which isohmicly or otherwise generated by these components. Indeed, it is wellknown that many electronic components, and especially semiconductorcomponents such as transistors and microprocessors, are more prone tofailure or malfunction at high temperatures. Thus, the ability todissipate heat often is a limiting factor on the performance of thecomponent.

[0005] Electronic components within integrated circuit traditionallyhave been cooled via forced or convective circulation of air within thehousing of the device. In this regard, cooling fins have been providedas an integral part of the component package or as separately attachedthereto for increasing the surface area of the package exposed toconvectively-developed air currents. Electric fans additionally havebeen employed to increase the volume of air which is circulated withinthe housing. For high power circuits and the smaller but more denselypacked circuits typical of current electronic designs, however, simpleair circulation often has been found to be insufficient to adequatelycool the circuit components.

[0006] Heat dissipation beyond that which is attainable by simple aircirculation may be effected by the direct mounting of the electroniccomponent to a thermal dissipation member such as a “cold plate” orother heat sink. The heat sink may be a dedicated, thermally-conductivemetal plate, or simply the chassis of the device. However, and as isdescribed in U.S. Pat. No. 4,869,954, the faying thermal interfacesurfaces of the component and heat sink typically are irregular, eitheron a gross or a microscopic scale. When the interfaces surfaces aremated, pockets or void spaces are developed therebetween in which airmay become entrapped. These pockets reduce the overall surface areacontact within the interface which, in turn, reduces the efficiency ofthe heat transfer therethrough. Moreover, as it is well known that airis a relatively poor thermal conductor, the presence of air pocketswithin the interface reduces the rate of thermal transfer through theinterface.

[0007] To improve the efficiency of the heat transfer through theinterface, a layer of a thermally-conductive material typically isinterposed between the heat sink and electronic component to fill in anysurface irregularities and eliminate air pockets. Initially employed forthis purpose were materials such as silicone grease or wax filled with athermally-conductive filler such as aluminum oxide. Such materialsusually are semi-liquid or sold at normal room temperature, but mayliquefy or soften at elevated temperatures to flow and better conform tothe irregularities of the interface surfaces.

[0008] For example, U.S. Pat. No. 4,299,715 discloses a wax-like,heat-conducting material which is combined with another heat-conductingmaterial, such as a beryllium, zinc, or aluminum oxide powder, to form amixture for completing a thermally-conductive path from a heated elementto a heat sink. A preferred wax-like material is a mixture of ordinarypetroleum jelly and a natural or synthetic wax, such as beeswax, palmwax, or mineral wax, which mixture melts or becomes plastic at atemperature above normal room temperature. The material can beexcoriated or ablated by marking or rubbing, and adheres to the surfaceon which it was rubbed. In this regard, the material may be shaped intoa rod, bar, or other extensible form which may be carried in apencil-like dispenser for application.

[0009] U.S. Pat. No. 4,466,483 discloses a thermally-conductive,electrically-insulating gasket. The gasket includes a web or tape whichis formed of a material which can be impregnated or loaded with anelectrically-insulating, heat conducting material. The tape or webfunctions as a vehicle for holding the meltable material and heatconducting ingredient, if any, in a gasket-like form. For example, acentral layer of a solid plastic material may be provided, both sides ofwhich are coated with a meltable mixture of wax, zinc oxide, and a fireretardant.

[0010] U.S. Pat. No. 4,473,113 discloses a thermally-conductive,electrically-insulating sheet for application to the surface of anelectronic apparatus. The sheet is provided as having a coating on eachside thereof a material which changes state from a solid to a liquidwithin the operating temperature range of the electronic apparatus. Thematerial may be formulated as a meltable mixture of wax and zinc oxide.

[0011] U.S. Pat. No. 4,764,845 discloses a thermally-cooled electronicassembly which includes a housing containing electronic components. Aheat sink material fills the housing in direct contact with theelectronic components for conducting heat therefrom. The heat sinkmaterial comprises a paste-like mixture of particulate microcrystallinematerial such as diamond, boron nitride, or sapphire, and a fillermaterial such as a fluorocarbon or paraffin.

[0012] The greases and waxes of the aforementioned types heretoforeknown in the art, however, generally are not self-supporting orotherwise form stable at room temperature and are considered to be messyto apply to the interface surface of the heat sink or electroniccomponent. To provide these materials in the form of a film which oftenis preferred for ease of handling, a substrate, web, or other carriermust be provided which introduces another interface layer in or betweenwhich additional air pockets may be formed. Moreover, use of suchmaterials typically involves hand application or lay-up by theelectronics assembler which increases manufacturing costs.

[0013] Alternatively, another approach is to substitute a cured,sheet-like material for the silicone grease or wax material. Suchmaterials may be compounded as containing one or morethermally-conductive particulate fillers dispersed within a polymericbinder, and may be provided in the form of cured sheets, tapes, pads, orfilms. Typical binder materials include silicones, urethanes,thermoplastic rubbers, and other elastomers, with typical fillersincluding aluminum oxide, magnesium oxide, zinc oxide, boron nitride,and aluminum nitride.

[0014] Exemplary of the aforesaid interface materials is an alumina orboron nitride-filled silicone or urethane elastomer which is marketedunder the name CHO-THERM® by the Chomerics Division of Parker-HannifinCorp., Woburn, Mass. Additionally, U.S. Pat. No. 4,869,954 discloses acured, form-stable, sheet-like, thermally-conductive material fortransferring thermal energy. The material is formed of a urethanebinder, a curing agent, and one or more thermally conductive fillers.The fillers may include aluminum oxide, aluminum nitride, boron nitride,magnesium oxide, or zinc oxide.

[0015] U.S. Pat. No. 4,782,893 discloses a thermally-conductive,electrically-insulative pad for placement between an electroniccomponent and its support frame. The pad is formed of a high dielectricstrength material in which is dispersed diamond powder. In this regard,the diamond powder and a liquid phase of the high dielectric strengthmaterial may be mixed and then formed into a film and cured. After thefilm is formed, a thin layer thereof is removed by chemical etching orthe like to expose the tips of the diamond particles. A thin boundarylayer of copper or other metal then is bonded to the top and bottomsurfaces of the film such that the exposed diamond tips extend into thesurfaces to provide pure diamond heat transfer paths across the film.The pad may be joined to the electronic component and the frame withsolder or an adhesive.

[0016] U.S. Pat. No. 4,965,699 discloses a printed circuit device whichincludes a memory chip mounted on a printed circuit card. The card isseparated from an associated cold plate by a layer of a siliconeelastomer which is applied to the surface of the cold plate.

[0017] U.S. Pat. No. 4,974,119 discloses a heat sink assembly whichincludes an electronic component supported on a printed circuit board ina spaced-apart relationship from a heat dispersive member. Athermally-conductive, elastomeric layer is interposed between the boardand the electronic component. The elastomeric member may be formed ofsilicone and preferably includes a filler such as aluminum oxide orboron nitride.

[0018] U.S. Pat. No. 4,979,074 discloses a printed circuit board devicewhich includes a circuit board which is separated from athermally-conductive plate by a pre-molded sheet of silicone rubber. Thesheet may be loaded with a filler such as alumina or boron nitride.

[0019] U.S. Pat. No. 5,137,959 discloses a thermally-conductive,electrically insulating interface material comprising a thermoplastic orcross linked elastomer filled with hexagonal boron nitride or alumina.The material may be formed as a mixture of the elastomer and filler,which mixture then may be cast or molded into a sheet or other form.

[0020] U.S. Pat. No. 5,194,480 discloses another thermally-conductive,electrically-insulating filled elastomer. A preferred filler ishexagonal boron nitride. The filled elastomer may be formed into blocks,sheets, or films using conventional methods.

[0021] U.S. Pat. Nos. 5,213,868 and 5,298,791 disclose athermally-conductive interface material formed of a polymeric binder andone or more thermally-conductive fillers. The fillers may be particulatesolids, such as aluminum oxide, aluminum nitride, boron nitride,magnesium oxide, or zinc oxide. The material may be formed by casting ormolding, and preferably is provided as a laminated acrylic pressuresensitive adhesive (PSA) tape. At least one surface of the tape isprovided as having channels or through-holes formed therein for theremoval of air from between that surface and the surface of a substratesuch as a heat sink or an electronic component.

[0022] U.S. Pat. No. 5,321,582 discloses an electronic component heatsink assembly which includes a thermally-conductive laminate formed ofpolyamide which underlays a layer of a boron nitride-filled silicone.The laminate is interposed between the electronic component and thehousing of the assembly.

[0023] Sheet-like materials of the above-described types have garneredgeneral acceptance for use as interface materials in conductively-cooledelectronic component assemblies. For some applications, however, heavyfastening elements such as springs, clamps, and the like are required toapply enough force to conform these materials to the interface surfacesto attain enough surface for efficient thermal transfer. Indeed, forcertain applications, materials such as greases and waxes which liquefy,melt, or soften at elevated temperature sometimes are preferred asbetter conforming to the interface surfaces. It therefore will beappreciated that further improvements in these types of interfacematerials and methods of applying the same would be well-received by theelectronics industry. Especially desired would be a thermal interfacematerial which is self-supporting and form-stable at room temperature,but which is softenable or meltable at temperatures within the operatingtemperature range of the electronic component to better conform to theinterface surfaces.

BROAD STATEMENT OF THE INVENTION

[0024] The present invention is directed to a heat transfer materialwhich is interposable between the thermal interfaces of aheat-generating, electronic component and a thermal dissipation member.The material is of the type which melts or softens at a temperature orrange within the operating temperature range of the electronic componentto better conform to the thermal interfaces for improved heat transferfrom the electronic component to the thermal dissipation member. Unlikethe greases or waxes of such type heretofore known in the art, however,the interface material of the present invention is form-stable andself-supporting at room temperature. Accordingly, the material may beformed into a film or tape which may be applied using automatedequipment to, for example, the interface surface of a thermaldissipation member such as a heat sink. In being self-supporting, no webor substrate need be provided which would introduce another layer intothe interface between which additional air pockets could be formed.

[0025] It therefore is a feature of the present invention to provide forthe conductive cooling a heat-generating electronic component. Thecomponent has an operating temperature range above normal roomtemperature and a first heat transfer surface disposable in thermaladjacency with a second heat transfer surface of an associated thermaldissipation member to define an interface therebetween. Athermally-conductive material is provided which is form-stable at normalroom temperature in a first phase and conformable in a second phase tosubstantially fill the interface. The material, which has a transitiontemperature from the first phase to the second phase within theoperating temperature range of the electronic component, is formed intoa self-supporting layer. The layer is applied to one of the heattransfer surfaces, which surfaces then are disposed in thermal adjacencyto define the interface. The energization of the electronic component iseffective to heat the layer to a temperature which is above the phasetransition temperature.

[0026] It is a further feature of the invention to provide athermally-conductive interface for conductively cooling aheat-generating electronic component having an associated thermaldissipation member such as a heat sink. The interface is formed as aself-supporting layer of a thermally-conductive material which isform-stable at normal room temperature in a first phase andsubstantially conformable in a second phase to the interface surfaces ofthe electronic component and thermal dissipation member. The materialhas a transition temperature from the first phase to the second phasewhich is within the operating temperature range of the electroniccomponent.

[0027] Advantages of the present invention include a thermal interfacematerial which melts or softens to better conform to the interfacessurfaces, but which is self-supporting and form-stable at roomtemperature for ease of handling and application. Further advantagesinclude an interface material which may be formed into a film or tapewithout a web or other supporting substrate, and which may be appliedusing automated methods to, for example, the interface surface of athermal dissipation member. Such member then may be shipped to amanufacturer for direct installation into a circuit board to therebyobviate the need for hand lay-up of the interface material. Stillfurther advantages include a thermal interface formulation which may betailored to provide controlled thermal and viscometric properties. Theseand other advantages will be readily apparent to those skilled in theart based upon the disclosure contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For a fuller understanding of the nature and objects of theinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein:

[0029]FIG. 1 is a fragmentary, cross-sectional view of an electricalassembly wherein a heat-generating electronic component thereof isconductively cooled in accordance with the present invention via theprovision of an interlayer of a thermally-conductive material within thethermal interface between the heat transfer surfaces of the componentand an associated thermal dissipation member;

[0030]FIG. 2 is a view of a portion of the thermal interface of FIG. 1which is enlarged to detail the morphology thereof;

[0031]FIG. 3 is a cross-sectional end view which shows thethermally-conductive material of FIG. 1 as coated as a film layer onto asurface of a release sheet, which sheet is rolled to facilitate thedispensing of the film; and

[0032]FIG. 4 is a view of a portion of the film and release sheet rollof FIG. 3 which is enlarged to detail the structure thereof.

[0033] The drawings will be described further in connection with thefollowing Detailed Description of the Invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Referring to the drawings wherein corresponding referencecharacters indicate corresponding elements throughout the figures, showngenerally at 10 in FIG. 1 is an electrical assembly which includes aheat-generating digital or analog electronic component, 12, supported onan associated printed circuit board (PCB) or other substrate, 14.Electrical component 12 may be an integrated microchip, microprocessor,transistor, or other semiconductor, or an ohmic or other heat-generatingsubassembly such as a diode, relay, resistor, transformer, amplifier,diac, or capacitor. Typically, component 12 will have an operatingtemperature range of from about 60-80° C. For the electrical connectionof component 12 to board 14, a pair of leads or pins, 16 a and 16 b, areprovided as extending from either end of component 12 into a soldered orother connection with board 14. Leads 16 additionally may supportcomponent 12 above board 14 to define a gap, represented at 17, of about3 mils (75 microns) therebetween. Alternatively, component 12 may bereceived directly on board 14.

[0035] As supported on board 14, electronic component 12 presents afirst heat transfer surface, 18, which is disposable in a thermal,spaced-apart adjacency with a corresponding second heat transfersurface, 22, of an associated thermal dissipation member, 20.Dissipation member 20 is constructed of a metal material or the likehaving a heat capacity relative to that of component 12 to be effectiveis dissipating thermal energy conducted or otherwise transferredtherefrom. For purposes of the present illustration, thermal dissipationmember 20 is shown as a heat sink having a generally planar baseportion, 24, from which extends a plurality of cooling fins, one ofwhich is referenced at 26. With assembly 10 configured as shown, fins 26assist in the convective cooling of component 12, but alternatively maybe received within an associated cold plate or the like, not shown, forfurther conductive dissipation of the thermal energy transferred fromcomponent 12.

[0036] The disposition of first heat transfer surface 18 of electroniccomponent 12 in thermal adjacency with second heat transfer surface 22of dissipation member 20 defines a thermal interface, represented at 28,therebetween. A thermally-conductive interlayer, 30, is interposedwithin interface 28 between heat transfer surfaces 18 and 22 forproviding a conductive path therethrough for the transfer of thermalenergy from component 12 to dissipation member 20. Such path may beemployed without or in conjunction with convective air circulation foreffecting the cooling of component 12 and ensuring that the operatingtemperature thereof is maintained below specified limits.

[0037] Although thermal dissipation member 20 is shown to be a separateheat sink member, board 14 itself may be used for such purpose byalternatively interposing interlayer 30 between surface 32 thereof andcorresponding surface 34 of electronic component 12. In eitherarrangement, a clip, spring, or clamp or the like (not shown)additionally may be provided for applying an external force, representedat 32, of from about 1-2 lbs_(f) for improving the interface areacontact between interlayer 30 and surfaces 18 and 22 or 32 and 34.

[0038] In accordance with the precepts of the present invention,interlayer 30 is formed of a self-supporting film, sheet, or other layerof a thermally-conductive material. By “self-supporting” it is meantthat interlayer 30 is free-standing without the support of a web orsubstrate which would introduce another layer into the thermal interfacebetween air pockets could be formed. Typically, the film or sheet ofinterlayer 30 will have a thickness of from about 1-10 mils (25-250microns) depending upon the particular geometry of assembly 10.

[0039] The thermally-conductive material forming interlayer 30 isformulated to be form-stable at normal room temperature, i.e., about 25°C., in a first phase, which is solid, semi-solid, glassy, orcrystalline, but to be substantially conformable in a second phase,which is a liquid, semi-liquid, or otherwise viscous melt, to interfacesurfaces 18 and 22 of, respectively, electronic component 12 and thermaldissipation member 20. The transition temperature of the material, whichmay be its melting or glass transition temperature, is preferably fromabout 60 or 70° C. to about 80° C., and is tailored to fall within theoperating temperature of electronic component 12.

[0040] Further in this regard, reference may be had to FIG. 2 wherein anenlarged view of a portion of interface 28 is illustrated to detail theinternal morphology thereof during the energization of electroniccomponent 12 effective to heat interlayer 30 to a temperature which isabove its phase transition temperature. Interlayer 30 accordingly isshown to have been melted or otherwise softened from a form-stable solidor semi-solid phase into a flowable or otherwise conformable liquid orsemi-liquid viscous phase which may exhibit relative intermolecularchain movement. Such viscous phase provides increased surface areacontact with interface surfaces 18 and 22, and substantially completelyfills interface 28 via the exclusion of air pockets or other voidstherefrom to thereby improve both the efficiency and the rate of heattransfer through interface. Moreover, as depending on, for example, themelt flow index or viscosity of interlayer 30 and the magnitude of anyapplied external pressure 36 (FIG. 1), the interface gap betweensurfaces 18 and 22 may be narrowed to further improve the efficiency ofthe thermal transfer therebetween. Any latent heat associated with thephase change of the material forming interlayer 30 additionallycontributes to the cooling of component 12.

[0041] Unlike the greases or waxes of such type heretofore known in theart, however, interlayer of the present invention advantageously isform-stable and self-supporting at room temperature. Accordingly, and asis shown generally at 40 in FIG. 3, interlayer 30 advantageously may beprovided in a rolled, tape form to facilitate its application to thesubstrate by an automated process. As may be better appreciated withadditional reference to FIG. 4 wherein a portion, 42, of tape 40 isshown in enhanced detail, tape 40 may be formed by applying a film ofinterlayer 30 to a length of face stock, liner, or other release sheet,44. Interlayer 30 may be applied to a surface, 46, of release sheet 44in a conventional manner, for example, by a direct process such asspraying, knife coating, roller coating, casting, drum coating, dipping,or like, or an indirect transfer process utilizing a silicon releasesheet. A solvent, diluent, or other vehicle may be provided to lower theviscosity of the material forming interlayer 30. After the material hasbeen applied, the release sheet may be dried to flash the solvent andleave an adherent, tack-free film, coating, or other residue of thematerial thereon.

[0042] As is common in the adhesive art, release sheet 44 may beprovided as a strip of a waxed, siliconized, or other coated paper orplastic sheet or the like having a relatively low surface energy so asto be removable without appreciable lifting of interlayer 30 from thesubstrate to which it is ultimately applied. Representative releasesheets include face stocks or other films of plasticized polyvinylchloride, polyesters, cellulosics, metal foils, composites, and thelike.

[0043] In the preferred embodiment illustrated, tape 40 may be sectionedto length, and the exposed surface, 48, of interlayer 30 may be appliedto interface surface 22 of dissipation member 20 (FIG. 1) prior to itsinstallation in assembly 10. In this regard, interlayer exposed surface48 may be provided as coated with a thin film of a pressure sensitiveadhesive or the like for adhering interlayer 30 to dissipation member20. Alternatively, interface surface 22 of dissipation member 20 may beheated to melt a boundary layer of interlayer surface 48 for itsattachment via a “hot-melt” mechanism.

[0044] With tape 40 so applied and with release sheet 44 protecting theunexposed surface, 50, of interlayer 30, dissipation member 20 (FIG. 1)may be packaged and shipped as an integrated unit to an electronicsmanufacturer, assembler, or other user. The user then simply may removerelease sheet 44 to expose surface 50 of interlayer 30, position surface50 on heat transfer surface 18 of electronic component 12, and lastlyapply a clip or other another means of external pressure to disposeinterlayer surface 50 in an abutting, heat transfer contact or otherthermal adjacency with electronic component surface 18.

[0045] In one preferred embodiment, interlayer 30 is formulated as aform-stable blend of: (a) from about 25 to 50% by weight of a pressuresensitive adhesive (PSA) component having a melting temperature of fromabout 90-100° C.; (b) from about 50 to 75% by weight of an α-olefinic,thermoplastic component having a melting temperature of from about50-60° C.; and (c) from about 20 to 80% by weight of one or morethermally-conductive fillers. “Melting temperature” is used herein inits broadest sense to include a temperature or temperature rangeevidencing a transition from a form-stable solid, semi-solid,crystalline, or glassy phase to a flowable liquid, semi-liquid, orotherwise viscous phase or melt which may be characterized as exhibitingintermolecular chain rotation.

[0046] The PSA component generally may be of an acrylic-based, hot-meltvariety such as a homopolymer, copolymer, terpolymer, interpenetratingnetwork, or blend of an acrylic or (meth)acrylic acid, an acrylate suchas butyl acrylate, and/or an amide such as acrylamide. The term “PSA” isused herein in its conventional sense to mean that the component isformulated has having a glass transition temperature, surface energy,and other properties such that it exhibits some degree of tack at normalroom temperature. Acrylic hot-melt PSAs of such type are marketedcommercially by Heartland Adhesives, Germantown, Wis., under the tradedesignations “H600” and “H251”.

[0047] The α-olefinic thermoplastic component preferably is a polyolefinwhich may be characterized as a “low melt” composition. A representativematerial of the preferred type is an amorphous polymer of a C₁₀ orhigher alkene which is marketed commercially by Petrolite Corporation,Tulsa, Okla., under the trade designation “Vybar® 260.” Such materialmay be further characterized as is set forth in Table 1. TABLE 1Physical Properties of Representative Olefinic Polymer Component(Vybar ® 260) Molecular Weight 2600 g/mol Melting Point (ASTM D 36) 130°F. (54° C.) Viscosity (ASTM D 3236) @ 210° F. (99° C.) 357.5 cPPenetration (ASTM D 1321) @ 77° F. (25° C.) 12 mm Density (ASTM D 1168)0.90 g/cm³ @ 75° F. (24° C.) @ 200° F. (93° C.) 0.79 g/cm³ Iodine Number(ASTM D 1959) 15

[0048] By varying the ratio within the specified limits of the PSA tothe thermoplastic component, the thermal and viscometric properties ofthe interlayer formulation may be tailored to provide controlled thermaland viscometric properties. In particular, the phase transitiontemperature and melt flow index or viscosity of the formulation may beselected for optimum thermal performance with respect to such variablesas the operating temperature of the heat generating electroniccomponent, the magnitude of any applied external pressure, and theconfiguration of the interface.

[0049] In an alternative embodiment, a paraffinic wax or other naturalor synthetic ester of a long-chain (C₁₆ or greater) carboxylic acid andalcohol having a melting temperature of from about 60-70° C. may besubstituted for the thermoplastic and PSA components to comprise about20-80% by weight of the formulation. A preferred wax is marketedcommercially by Bareco Products of Rock Hill, S.C. under the tradedesignation “Ultraflex® Amber,” and is compounded as a blend ofclay-treated microcrystalline and amorphous constituents. Such wax isadditionally characterized in Table 2 which follows. TABLE 2 PhysicalProperties of Representative Paraffinic Wax Component (Ultraflex ®Amber) Melting Point (ASTM D 127) 156° F. (69° C.) Viscosity (ASTM D3236) @ 210° F. (99° C.) 13 cP Penetration (ASTM D 1321) @ 77° F. (25°C.) 29 mm @ 110° F. (43° C.) 190 mm Density (ASTM D 1168) @ 77° F. (25°C.) 0.92 g/cm³ @ 210° F. (99° C.) 0.79 g/cm³

[0050] In either of the described embodiments, the resin or waxcomponents form a binder into which the thermally-conductive filler isdispersed. The filler is included within the binder in a proportionsufficient to provide the thermal conductivity desired for the intendedapplication. The size and shape of the filler is not critical for thepurposes of the present invention. In this regard, the filler may be ofany general shape including spherical, flake, platelet, irregular, orfibrous, such as chopped or milled fibers, but preferably will be apowder or other particulate to assure uniform dispersal and homogeneousmechanical and thermal properties. The particle size or distribution ofthe filler typically will range from between about 0.25-250 microns(0.01-10 mils), but may further vary depending upon the thickness ofinterface 28 and/or interlayer 30.

[0051] It additionally is preferred that the filler is selected as beingelectrically-nonconductive such that interlayer 30 may provide anelectrically-insulating but thermally-conductive barrier betweenelectronic component 12 and thermal dissipation member 20. Suitablethermally-conductive, electrically insulating fillers include boronnitride, alumina, aluminum oxide, aluminum nitride, magnesium oxide,zinc oxide, silicon carbide, beryllium oxide, and mixtures thereof. Suchfillers characteristically exhibit a thermal conductivity of about 25-50W/m-K.

[0052] Additional fillers and additives may be included in interlayer 30to the extent that the thermal conductivity and other physicalproperties thereof are not overly compromised. As aforementioned, asolvent or other diluent may be employed during compounding to lower theviscosity of the material for improved mixing and delivery. Conventionalwetting opacifying, or anti-foaming agents, pigments, flame retardants,and antioxidants also may be added to the formulation depending upon therequirements of the particular application envisioned. The formulationmay be compounded in a conventional mixing apparatus.

[0053] Although not required, a carrier or reinforcement member (notshown) optionally may be incorporated within interlayer 30 as a separateinternal layer. Conventionally, such member may be provided as a filmformed of a thermoplastic material such as a polyimide, or as a layer ofa woven fiberglass fabric or an expanded aluminum mesh. Thereinforcement further supports the interlayer to facilitate its handlingat higher ambient temperatures and its die cutting into a variety ofgeometries.

[0054] The Example to follow, wherein all percentages and proportionsare by weight unless otherwise expressly indicated, is illustrative ofthe practicing of the invention herein involved, but should not beconstrued in any limiting sense.

EXAMPLE

[0055] Master batches representative of the interlayer formulations ofthe present invention were compounded for characterization according tothe following schedule: TABLE 3 Representative Interlayer FormulationsUltraflex ® Sample Vybar ® 260¹ H600² Amber³ Filler (wt. %) No. (wt. %)(wt. %) (wt. %) BN⁴ ZnO₂ ⁵ Al⁶ 3-1  45 22 33 3-2  47 17 36 3-3  47 17 630 3-6  40 60 3-7  40 19 41 3-8  50 25 25 3-10 34 16 50 5-1  67 33

[0056] The Samples were thinned to about 30-70% total solids withtoluene or xylene, cast, and then dried to a film thickness of fromabout 2.5 to 6 mils. When heated to a temperature of between about55-65° C., the Samples were observed to exhibit a conformable grease orpaste-like consistency. The following thermal properties were measuredand compared with conventional silicone grease (Dow 340, Dow Corning,Midland, Mich.) and metal foil-supported wax (Crayotherm®, CrayothermCorp., Anaheim, Calif.) formulations: TABLE 4 Thermal Properties ofRepresentative and Comparative Interlaver Formulations Thermal ThermalSample Impedance⁵ Conductivity⁵ No. Formulation Filler (wt. %) Thickness(mils) (° C.-in/w) (w/m-K) 3-1  blend¹ 62% Al 6 0.14 1.7 3-2  blend 62%Al 4 0.12 1.3 3-3  blend 62% Al/BN 4 0.09 1.7 3-6  wax² 60% Al 2.5 0.042.3 5-1  wax 50% BN 4 0.10 1.5 3-7  blend 62% ZnO₂ 4 0.14 1.1 3-8  blend30% BN 2.5 0.07 1.5 3-10 blend 70% ZnO₂ 3 0.12 0.95 Crayotherm wax/foil³ZnO₂ 2.5 0.11 0.93 3-2  blend 62% Al 5 0.26 0.74 3-6  wax 60% Al 5 0.300.65 5-1  wax 50% BN 5 0.12 1.64 Dow 340 grease⁴ ZnO₂ 5 (true⁶) 0.360.54

[0057] The foregoing results confirm that the interlayer formulations ofthe present invention retain the preferred conformal and thermalproperties of the greases and waxes heretofore known in the art.However, such formulations additionally are form-stable andself-supporting at room temperature, thus affording easier handling andapplication and obviating the necessity for a supporting substrate, web,or other carrier.

[0058] As it is anticipated that certain changes may be made in thepresent invention without departing from the precepts herein involved,it is intended that all matter contained in the foregoing descriptionshall be interpreted as illustrative and not in a limiting sense. Allreferences cited herein are expressly incorporated by reference.

What is claimed is:
 1. A thermal interface material which undergoes aphase change at microprocessor operating temperatures to transfer heatgenerated by a heat source to a heat sink, the material comprising: aphase change substance which softens at about the operating temperatureof the heat source, the phase change substance including: a polymercomponent, and a melting point component mixed with the polymercomponent, which modifies the temperature at which the phase changesubstance softens, the melting point component melting at around themicroprocessor operating temperatures and dissolving the polymercomponent in the melting point component; and a thermally conductivefiller dispersed within the phase change substance.
 2. The thermalinterface material of claim 1, wherein the phase change substance has aviscosity of from 1 to 100 poise at the operating temperature of theheat source.
 3. The thermal interface material of claim 1, wherein thephase change substance has a viscosity of from 5 to 50 poise in thetemperature range of 60 to 120° C.
 4. The thermal interface material ofclaim 1, wherein the phase change substance has a melting point of30-120° C.
 5. The thermal interface material of claim 1, wherein thepolymer component includes an elastomer selected from the groupconsisting of silicone, acrylic polymers, natural rubber, syntheticrubber, and combinations thereof.
 6. The thermal interface material ofclaim 1, wherein the polymer component has a Mooney viscosity of up to40 ML4.
 7. A thermal interface material which undergoes a phase changeat microprocessor operating temperatures to transfer heat generated by aheat source to a heat sink, the material comprising: a phase changesubstance which softens at about the operating temperature of the heatsource, the phase change substance including: a polymer component, and amelting point component mixed with the polymer component, which modifiesthe temperature at which the phase change substance softens, the meltingpoint component being selected from the group consisting of C₁₂-C₁₆alcohols, acids, esters, petroleum waxes, wax-like compounds, lowmolecular weight styrenes, methyl triphenyl silane materials, andcombinations thereof; and a thermally conductive filler dispersed withinthe phase change substance.
 8. The thermal interface material of claim7, wherein the melting point component is a C₁₂-C₁₆ alcohol or acidselected from the group consisting of myristyl alcohol, cetyl alcohol,stearyl alcohol, myristyl acid, stearic acid, and combinations thereof.9. The thermal interface material of claim 7, wherein the melting pointcomponent is a wax or a waxlike compound selected from the groupconsisting of microcrystalline wax, paraffin waxes, cyclopentane,heceicosyl, 2-heptadecanone, pentacosaneyl, silicic acid, tetraphenylester, octadecanoic acid, 2-[2-[2-(2hydroxyethoxy) ethoxy]ethoxy]ethylester, cyclohexane docosyl, polystyrene, polyamide resins, disiloxane1,1,1, trimethyl-3,3, triphenyl silane, and combinations thereof. 10.The thermal interface material of claim 1, wherein the polymer componenthas a solubility parameter which is within +1 and −1 of the solubilityparameter of the melting point component.
 11. The thermal interfacematerial of claim 1, wherein: the polymer component is at aconcentration of from 10-80% by weight; the filler is at a concentrationof from 10-80% by weight; and the melting point component is at aconcentration of from 10-80% by weight.
 12. The thermal interfacematerial of claim 11, wherein: the polymer component is at aconcentration of from 10-70% by weight; the filler is at a concentrationof from 10-70% by weight; and the melting point component is at aconcentration of from 15-70% by weight.
 13. The thermal interfacematerial of claim 1, wherein the thermally conductive filler has a bulkthermal conductivity of between about 0.5 and 1000 watts meter perdegree Kelvin.
 14. The thermal interface material of claim 1, whereinthe thermal interface material has a thermal conductivity of at least0.8 watts meter per degree Kelvin.
 15. The thermal interface material ofclaim 1, wherein the thermally conductive filler is selected from thegroup consisting of boron nitride, aluminum oxide, nickel powder, copperflakes, graphite powder, powdered diamond, and combinations thereof. 16.The thermal interface material of claim 1, wherein the thermallyconductive filler has an average particle size of from about 2 to 100microns.
 17. A thermal interface material comprising: a polymercomponent at a concentration of from 10-80% by weight of the material,the polymer component including an elastomer; a melting point componentat a concentration of from 10-80% by weight of the material, the meltingpoint component including a wax or a waxlike compound, the polymercomponent having a solubility parameter which is within +1 and −1 of thesolubility parameter of the melting point component; and a thermallyconductive filler at a concentration of from 10-80% by weight of thematerial.
 18. The thermal interface material of claim 17, wherein theelastomer is selected from the group consisting of silicone, acrylicpolymers, natural rubber, synthetic rubber, and combinations thereof.19. The thermal interface material of claim 17, wherein the meltingpoint component includes a compound selected from the group consistingof microcrystalline wax, paraffin waxes, cyclopentane, heceicosyl,2-heptadecanone, pentacosaneyl, silicic acid, tetraphenyl ester,octadecanoic acid, 2-[2-[2-(2hydroxyethoxy) ethoxy]ethoxy]ethyl ester,cyclohexane docosyl, polystyrene, polyamide resins, disiloxane 1,1,1,trimethyl-3,3, triphenyl silane, C₁₂-C₁₆ alcohols, C₁₂-C₁₆ acids, andcombinations thereof.
 20. The thermal interface material of claim 1,wherein the melting point component includes a compound selected fromthe group consisting of microcrystalline wax, paraffin waxes,cyclopentane, heceicosyl, 2-heptadecanone, pentacosaneyl, silicic acid,tetraphenyl ester, octadecanoic acid, 2-[2-[2-(2hydroxyethoxy)ethoxy]ethoxy]ethyl ester, cyclohexane docosyl, polystyrene, polyamideresins, disiloxane 1,1,1, trimethyl-3,3, triphenyl silane, C₁₂-C₁₆alcohols, C₁₂-C₁₆ acids, and combinations thereof.